With the development of a recent mobile communication system, the communication system requires a wideband, high frequency and high reliability. Accordingly, transmit diversity technologies are effective in improving transmission quality without increasing a radio part of a terminal. Moreover, it is known that two-dimensional spreading capable of increasing a spreading gain is effective in an environment in which other-cell interference is serious.
Time-domain spreading is considered as orthogonal frequency-division multiplexing-code-division multiplexing (OFDM-CDM)-based spreading, because the orthogonality between frequency-domain spreading codes is lower than the orthogonality between time-domain spreading codes in a frame format in which amplitude variation in a time domain through Doppler frequency according to the movement of a terminal is less than amplitude variation in a frequency domain according to frequency selectivity of a transfer path. Where a large spreading factor is required, two-dimensional spreading based on both the time and frequency domains is used. When a de-spreading operation is carried out, amplitude and phase in each subcarrier are compensated using a channel response in each subcarrier.
The time domain spreading is disclosed in “Properties of OFDM-CDMA Systems Using Transmit Diversity in Reception Link”, Incheol JEONG and Masao NAKAGAWA, IEICE Technical Report, RCS 2000-184, January 2000, and “A Study on Time-Domain Spreading in FCDM”, Kenichi MIYOSHI, Atushi MATSUMOTO and Mitsuru UESUGI, IEICE Technical Report, RCS 2001-179, November 2001.
The configurations of a wireless receiver and transmitter to which space-time transmit diversity is applied according to the OFDM-CDM based on the time-domain spreading are shown in FIGS. 10 and 11.
When transmission signals Ω are inputted, a wireless transmitting side carries out an encoding operation based on the following Equation 1.
                    Ω        =                  [                                                                      S                  1                                                                              S                  2                                                                                                      -                                      S                    2                    *                                                                                                S                  1                  *                                                              ]                                    Equation        ⁢                                  ⁢        1            
Two code streams [S1,−S2*] and [S2,S1*] as outputs of space-time encoding operations are outputted to antenna branches #1 and #2, respectively. The antenna branch #1 receives the code stream [S1,−S2*] and carries out a time-domain spreading operation for S1 and −S2* using one self-user signal spreading code as shown in FIG. 12. At this point, S1 is assigned to a 1st time slot, and −S2* is assigned to a 2nd time slot later than the 1st time slot. That is, space-time code signals of two different time slots are spread by the same self-user signal spreading code. Furthermore, the antenna branch #2 receives the code stream [S2,S1*] and carries out a time-domain spreading operation for S2 and S1* using the self-user signal spreading code as in the antenna branch #1.
At this point, S2 is assigned to the 1st time slot and S1* is assigned to the 2nd time slot later than the 1st time slot. That is, space-time code signals of two different time slots are spread by the same self-user signal spreading code.
Then, each of the antenna branches #1 and #2 multiplexes other user signals obtained similarly to the user signal spread by the self-user signal spreading code according to the time-domain spreading.
Moreover, each of the antenna branches #1 and #2 multiplexes the multiplexed self-user and other-user signals and a pilot signal pre-stored in the wireless transmitter and receiver.
The multiplexed signals containing the user and pilot signals are converted into time domain signals based on inverse fast Fourier transform (IFFT) and guard intervals (GIs) are added to the time-domain signals.
Antennas of the antenna branches #1 and #2 simultaneously radiate output signals after the GIs are added.
Therefore, when the conventional wireless transmitter applies space-time transmit diversity to a frame format, space-time encoding outputs shown in FIG. 12 as described above are assigned to two consecutive spreading regions (=two different time slots).
On the other hand, the antennas of the antenna branches #1 and #2 in the wireless receiver receive the signals radiated from the antennas of the antennas branches #1 and #2, and output the received signals to time-domain de-spreaders and channel estimators.
The channel estimators estimate channel responses from the signals received by the antennas using pre-stored pilot signals.
The time-domain de-spreaders subtract the pilot signals from the received signals and sequentially de-spread signals of two different time slots using the self-user signal spreading code.
Furthermore, a space-time decoder receives channel estimation values of channels h1 and h2, and obtains decoded signals by carrying out a space-time decoding operation for space-time code signals of two time slots consecutive in the time domain of the signals de-spread by the time-domain de-spreaders.
In an example described above, signals of the two different time slots are sequentially de-spread using the self-user signal spreading code so that a spreading encoding operation can be carried out using the same self-user signal spreading code between the space-time code signals of different time slots. For this reason, a channel response cannot be obtained symbol by symbol in the time de-spreading operation. Accordingly, the de-spreading operation in the wireless receiver corresponds to equivalent-gain combining de-spreading using only the self-user signal spreading code. There is a problem in that a spreading factor is limited so that the orthogonality between codes is maintained and the effect of time-domain variation is not present.
For example, when a 2×2 space-time code matrix is used where space-time transmit diversity is applied, two symbols outputted in the time domain are spread by two spreading regions in the time domain. Moreover, channel responses need to be invariable in time slot intervals of a plurality of symbols in relation to space-time codes.
Accordingly, a need exists for a design immune to the time-domain variation in two spreading regions. Furthermore, there is another problem in that design requirements are complex or transmission characteristics are degraded when two spreading regions are affected by the time variation.